Injectors with different types of automatic functions have been developed under a number of years. The functions could comprise penetration, injection, dose setting, mixing, priming, needle shields, just to mention a few.
One type of injectors is the one that is reusable, and/or capable of housing different sizes of medicament cartridges. In this aspect, and in particular when the injector should be capable to deliver a set dose, it is important that the plunger rod or the like acting on the cartridge for expelling a dose of medicament should be in the proper position in relation to the cartridge when the first dose is to be delivered from a new cartridge. This position should preferably be set automatically by the device, i.e. the user-actions should be kept to a minimum in order to minimize the risk of wrong handling of the device.
One such solution of automatically adjusting the position of the plunger rod in relation to a stopper of a medicament cartridge is disclosed in patent No. EP 1 218 042. It describes a medication delivery pen comprising a body wherein a threaded plunger rod or lead screw acting on a stopper of a medicament cartridge is arranged inside said body. The lead screw is provided with a spring at its proximal end for biasing the lead screw in the distal direction, i.e. towards the stopper. The lead screw and the spring are arranged inside a tubular driver. In this manner, when a medicament cartridge is inserted in a cartridge holder and mounted to the body, the lead screw is constantly pushed against the stopper until the cartridge holder is locked against the rest of the body, wherein the movement of the lead screw is also locked in the proper position in relation to the stopper. The device is thus “reset” and since there is no play between the lead screw and the stopper, the injector pen delivers the right, preset dose, from the first injection.
The design however requires that the pen injector becomes somewhat longer than without the spring, since it is arranged at the proximal end of the lead screw. Further, since the spring is held in place inside the driver, thereby preventing it from buckling or bulging, the lead screw has to have the corresponding diameter in order to fit inside the driver. The rather small diameter reduces the precision of the pitch of the threads, which in turn reduces the precision of the dose quantity delivered in relation to the preset dose.
There is thus room for improvements regarding automatic adjustment of drive units in relation to medicament cartridges in injectors.